The overall aim of this project is to expand our knowledge of the clinical features of MPS NIC and MPS HID disease by carrying out a prospective natural history study. We have identified 15 patients with MPS INC and 2 patients with MPSIIID. During the course of the study, we will determine the genotype and the residual enzyme activity, as well as do periodic measurements by neurocognitive testing, measure glycosaminoglycans in urine, observations of clinical status, and use brain imaging to document the course of the disease. Our hypothesis is that disease progression will be manifested by neurocognitive decline, and will correlate with genotype and/or residual enzyme activity, as well as with an increase in glycosaminoglycan levels in urine.